Climate Forcing and Ozone

FORECASTING UV DOSAGE AT THE EARTH'S SURFACE

The objective of accurately forecasting UV dosage levels has become far more important as the rate of climate forcing has accelerated – even prior to any release rate from carbon reservoirs in the Arctic - with the emerging results on carbon addition to the atmosphere (Raupach et al., 2007; Canadell et al., 2007), revealing that 2006 emission of carbon from fossil fuel combustion alone reached 8.4 GtC/yr and have held at or above those addition levels since. In addition, as the NRC Decadal Survey made clear, it is the irreversible nature of potential increases in water vapor in the stratosphere coupled with decreases in lower stratospheric temperatures resulting from CO₂ forcing and water vapor feedback that is of great concern, as these changes will increase the catalytic destruction of ozone in the stratosphere.

SUMMARY

This exponential response of the rate of conversion of ClONO₂ and HCl to free radical form to water vapor concentrations and temperature is displayed in Figure 2 linked to an observation of the water vapor concentration we have observed repeatedly in WB-57 missions over the US in the summer. The upper-most panel displays an H₂O profile of isotopically heavy water extending from 380 K to 440K reaching 15 ppm over an extended altitude domain. This concentration is linked into the temperature-water vapor plot for heterogeneous conversion of inorganic chlorine to free radical form. We have experimentally verified this threshold using trajectory tracing techniques demonstrating, as indicated by the dotted line, that this chemical conversion occurs rapidly (a few hours) at 195 K with 5 ppm water, increasing to > 200K at 15 ppm water. At > 200 K, this heterogeneous reaction is activated frequently in the lower stratosphere. This trigger point for conversion of inorganic chlorine to free radical form is quantitatively linked to the catalytic loss of ozone through the BrO + ClO rate limiting step as shown in the third panel of Figure 3 such that just a modest increase in ClO elevates the bromine-chlorine cycles to the dominant role in lower stratospheric ozone loss. There is significant evidence for this as Ross Salawitch has demonstrated using the data from Mt. Pinatubo showing that ozone loss cannot be quantitatively explained without the inclusion of bromine catalysis.

STRATEGY

It is a remarkable fact that perhaps the most important observation coupling climate forcing to UV dosage levels at the surface at mid-latitudes is the observation of high (e.g. > 10 ppmv) water vapor and low temperatures (< 210 K) with the simultaneous determination of BrO and ClO in the lower stratosphere at mid-latitudes. Yet this combination of observations has never been included in a mission strategy. We have a number of observations of high water and low temperatures at mid-latitudes. We have observations of BrO in the presence of low water. But we do not have BrO and ClO observations in the presence of high water vapor and low temperatures, which is the critical combination of conditions to appraise this climate-ozone feedback.

What is critically needed now is a mini-mission flown into the stratosphere over the United States in the summer to obtain observations of BrO, ClO, OH, HO₂, NO₂, H₂Ov, H₂Ot, HDO, CO₂, N₂O, and CO under conditions of high water. I think this would provide high impact for NASA at very limited cost, and it is in line with the mandated responsibility of NASA for stratospheric ozone. The idea that ozone is no longer a problem because of the Montreal Protocol is simply indefensible. If climate forcing significantly increases water vapor in the stratosphere, there is no way to reverse that secular trend. But we know from paleorecords that, at carbon dioxide levels above approximately 450–500 ppm, the stratosphere was probably considerably wetter than it is today (Sloan and Pollard, 1998). This results either from (1) changes in the overturning intensity of the Brewer-Dobsen system (Kirk-Davidoff et al., 2002) when forced by increasing CO₂, resulting in either changes in the boundary conditions of the tropical tropopause or convective pathways opening up for increasing flow of water into the stratosphere, or (2) an increased flux of methane that was oxidized in the stratosphere to form water that in turn triggers this structural change. The increase of water vapor in the stratosphere, even in limited amounts, constitutes a fundamental reframing of our understanding of the chemical-dynamical-radiative structure of Earth’s climate that is a climate “state” based in important ways on the assumption of a very dry stratosphere.

While past climates have had high water vapor concentrations in the stratosphere, the Earth system has never simultaneously had high water vapor concentrations and high halogen loading as may well be the case now under high levels of climate forcing by unprecedented levels of CO₂.